284 THE CANADIAN NATURALIST, [Vol, vi, 



subordinate beds of serpentine and granular limestone; the -whole 

 without organic remains. 



These crystalline rocks were, however, soon afterwards excluded 

 by him from the Cambrian series, for in 1838 [Proc. Geol. Soc. 

 II, 679] Sedojwick describes further the section from the Menai 

 Strait to the Berwyns, and assigns to the chloritic and micaceous 

 schists of Anglesea and Caernarvon a position inferior to the 

 Cambrian, which he divides into two parts; viz,, Lower Cambrian, 

 comprehending the old slate series, up to the Bala limestone 

 beds ; and Upper Cambrian, including the Bala beds and the 

 strata above them in the Berwyn chain, to which he gave the 

 name of the Bala group. The dividing line between the two 

 portions was subsequently extended downwards by Sedgwick to 

 the summit of the Arenig slates and porphyries. The lower 

 division was afterwards subdivided by him into the Bangor group, 

 (to which the name of Lower Cambrian was henceforth to be 

 restricted,) including the Llanberris roofing-slates and the Harlech 

 grits or Barmouth sandstones ; and the Festiniog group, which 

 included the Lingula-flags ^nd the succeeding Tremadoc slates. 



In the communication of Murchison to the same Dublin meet- 

 ing, in August, 1835, he repeated the description of the four 

 formations to which he had just given the nnme of Silurian; 

 which were, in descending order, Ludlow and Wenlock (Upper 

 Silurian), and Caradoc and Llandeilo (Lower Silurian). The 

 latter formation was then declared by Murchison to constitute 

 the base of the Silurian system, and to offer in many places in 

 South Wales distinct passages to the underlying slaty rocks, 

 which were, according to him, the Upper Cambrian of Sedgwick. 



Meanwhile, to go back to 1834, we find that after Murchison 

 had, in his communication to the Geological Society, defined the 

 relation of his Llandeilo formation to the underlying slaty series, 

 but before the names of Silurian and Cambrian had been given 

 to these respectively, Sedgwick and Murchison visited together the 

 principal sections of these rocks from Caermarthenshire to Den- 

 bighshire. The greater part of this region was then unknown to 

 Sedgwick, but had been already studied by Murchison, who in- 

 terpreted the sections to his companion in conformity with the 

 scheme already given ; according to which the beds of the Llan- 

 deilo were underlaid by the slaty rocks ^rhich appear along their 

 north-western border. When, however, they entered the region 

 which had already been examined by Sedgwick, and reached the 



